1. Field of the Invention
The invention relates to environmental and electromagnetic seals for rotary junctions. More specifically, this invention relates to combined environmental and electromagnetic rotary seals for electromagnetic energy transmission systems.
2. Description of the Related Art
For certain applications, it is necessary or desirable to provide an environmental as well as an electromagnetic seal. For example, in the field of directed energy, one system might involve transmission of a high-power millimeter wave beam. Due to the high power levels of these systems, seals are required to prevent leakage of electromagnetic energy. Such leakage can be deleterious to associated electronic devices and systems and personnel. The need for seals is exacerbated by the need for rotary joints to assist in the direction of energy. These joints must be sealed environmentally as well as electromagnetically.
Environmental seals serve to protect the joint against the introduction of dirt and debris which might adversely impact the performance of the host system and/or the electromagnetic seals.
Electromagnetic seals have a long history of usage in RF, microwave, and millimeter-wave systems. For example, a choke flange is an electromagnetic seal used with conventional waveguide to prevent leakage of electromagnetic energy from the junction between adjacent sections of waveguide into the environment. A choke flange prevents the passage of electromagnetic energy by presenting a discontinuity to incident electromagnetic fields, thereby reflecting them back in the direction from which they came. The discontinuity usually takes the form of one or more grooves cut into the face of the flange. A choke flange is an electromagnetic seal only. A separate environmental seal is required if contamination of the waveguide interior is to be prevented.
Gaskets are frequently used to provide environmental and/or electromagnetic seals. Non-conductive gaskets are largely transparent to incident electromagnetic radiation and thus provide an environmental seal only. Combined environmental-electromagnetic gaskets are available, but are expensive and fragile. Such gaskets work well when used to provide a seal between stationary components, e.g., between two waveguide flanges. When used as a seal between two components that can move with respect to one another, such gaskets are prone to wear due to friction.
Unlike standard environmental gaskets such as elastomeric o-rings whose performance is not degraded by lubrication, combination environmental-electromagnetic gaskets cannot be lubricated without at the same time seriously degrading their electrical performance. Due to their brittleness, they are damaged easily when handled or used in rotary applications and require frequent replacement. Moreover, combined environmental-electromagnetic gaskets of this type are prone to generating small free metal particles as they wear. This is problematic inasmuch as the presence of conductive particles in a high-power beam waveguide system can cause arcs that can degrade the beam transport path and/or damage expensive equipment.
Hence, a need exists in the art for an improved low-cost combined environmental-electromagnetic rotary seal for electromagnetic energy transmission systems.